Many of the defects found in PET container preforms can be detected by using the PL-2. Preforms viewed in polarized light exhibit a colored birefringence pattern. The birefringence is a result of the polymer flow lines (molecular orientational strain) produced during the injection moulding process. The colored pattern observed with preform held at 45° to the polarizing axis is known as an isochromatic fringe indicating the amount of birefringence.

Stress introduced during the manufacture of glass components has an important effect on the strength of the product. This effect can be detrimental or beneficial in that it may contribute to failure in service, or to an increase in strength. It is clearly important for quality control to monitor the residual stresses that are present.